The invention relates to a method and an arrangement for cooling the flow in radial gaps formed between rotors and stators of turbomachines, but in particular for cooling the flow in the radial gap between the compressor impeller and the casing of a radial compressor.
To seal off rotating systems, non-contact seals, in particular labyrinth seals, are widespread in turbomachine construction. In the separating gap, through which flow occurs, between rotating and stationary parts, high friction power occurs as a result of the forming flow boundary layers. This leads to heating of the fluid in the separating gap and thus also to the heating of the components surrounding the separating gap. The high material temperatures result in a reduction in the service life of the corresponding components.
DE 195 48 852 A1 discloses a radial compressor of simple construction without a sealing geometry formed in the separating gap. In this case, too, the friction heat produced as a result of flow shearing layers at the rear wall of the compressor impeller causes heating of the compressor impeller and thus reduces its service life.
EP 0 518 027 B1 discloses air cooling for radial compressors with a sealing geometry on the rear side of the compressor impeller. To this end, an additional annular space is formed between the individual sealing elements on the casing wall side of the radial compressor. A cold gas which has a higher pressure than the pressure prevailing at the outlet of the compressor impeller is directed into this annular space. The air supplied acts as impingement cooling. In the process, it divides in the sealing region and flows mainly radially inward as well as outward. This is intended to additionally achieve a blocking effect against the flow of hot compressor air through the separating gap from the outlet of the compressor impeller.
However, the cooling effect which can be achieved in this way is limited because of a number of factors. For example, blowing in air leads to an increase in the pressure and thrust, as a result of which the bearing loading increases. In addition, the temperature of the available air also represents a limiting element. In the case of high-speed compressor wheels in particular, and at high pressure ratios, such as are usual in modern turbocharger construction, it is therefore possible for situations to arise in which this type of cooling is inadequate.
In addition to direct cooling, DE 196 52 754 A1 also discloses indirect cooling of the rear wall of the compressor impeller or of the medium flowing through the separating gap. To this end, a feed and distributing device connected to the lubricating-oil system of the turbocharger is arranged on or in the casing part disposed at the rear wall and forming with the latter the separating gap. The oil used for the bearing lubrication serves as cooling medium, for which purpose the lubricating-oil circuit of the turbocharger is tapped. A disadvantage of this cooling is the relatively high oil demand and the heat quantity to be additionally dissipated by the oil cooler. This leads to an increased overall volume of the cooler. In addition, in the event of an accident with damage to the corresponding parts, there is an increased risk of deflagration. Just as in the case of direct cooling, the cooling effect which can be achieved with indirect cooling is also limited, the cause for which, in addition to the temperature of the cooling liquids which can be used in practice, can be determined as the low overall volume which is available.
The invention attempts to avoid all these disadvantages. Its object is to provide a method for cooling the flow in radial gaps formed between rotors and stators of turbomachines, which method is improved with regard to its cooling effect. In addition, a simple, cost-effective and robust arrangement for realizing the method is to be specified.
According to the invention, this is achieved in that, a first cooling fluid is admitted to a stator part adjacent to the radial gap and a second, gaseous cooling fluid is introduced into the radial gap.
Because of the use of a first cooling fluid for the indirect cooling and, additionally, a second cooling fluid for the direct cooling of the part flow, admitted to the radial gap, of the working medium of the turbomachine, a considerably improved cooling effect and also improved cooling effectiveness can be achieved. Thus, only this double cooling of the radial gap permits a further reduction in the temperature of the thermally severely loaded rotor into temperature ranges which could not be achieved with the conventional cooling configurations.
To this end, at least one recess is formed in the interior of the stator part adjacent to the radial gap or at least one cavity is arranged at the stator part. The recess or the cavity is connected to both a feed line and a discharge line for the first cooling fluid. In addition, at least one feed passage and a discharge device for the second cooling fluid are arranged at the radial gap.
It is particularly preferable for water to be used as the first cooling fluid and air as the second cooling fluid.
Water has a somewhat higher density than the known lubricating oils, and a specific heat capacity which is about twice as high. Since the flow of heat to be carried away via a cooling medium is proportional to the product of density and specific heat capacity, when water is used as the first cooling fluid, the result is a distinct advantage by comparison with oil cooling. Given the same mass flow and the same temperature of the water, it is therefore possible for a greater quantity of heat to be extracted from the medium flowing through the radial gap via the stator part to be cooled. The cooling effect on those regions of the rotor which adjoin the radial gap is therefore likewise greater. In the converse direction, in order to dissipate the same quantity of heat, a smaller mass flow of cooling water is needed by comparison with lubricating oil, as a result of which the feed and discharge device for the cooling fluid can be dimensioned correspondingly smaller.
Depending on the wall thickness on the rotor side, which is to be kept as small as possible, an improved cooling effect can be achieved by guiding the water directly adjacent to the radial gap in the interior of the stator part. If, however, instead of the recess in the stator part, the cavity described is formed at the stator part, simpler and more cost-effective manufacture can be realized, likewise with a good cooling effect.
The use of air as second cooling fluid proves to be particularly advantageous, since it is available both in the environment and in the turbomachine itself in an adequate quantity, at an adequate pressure and with a suitably low temperature.
In a system consisting of an internal combustion engine, a charge-air cooler and an exhaust-gas turbocharger, either fresh water from outside the system or advantageously water present in the system is used as first cooling fluid. In the latter case, the cooling water located in a cooling-water circuit of the charge-air cooler is use or this purpose, and this cooling water is branched off upstream of the charge-air cooler. In this case, the fixed stator part is a casing part of a radial compressor, and this casing part bounds the radial gap relative to the rotor, that is to say relative to the rotating compressor impeller of an exhaust-gas turbocharger.
If, on the other hand, oil is used as first cooling fluid, then this can advantageously be branched off from the lubricating-oil system present in any case in the bearing casing of the turbo machine. In this way, a relatively simpler and therefore cost-effective arrangement can be produced. If the first cooling fluid is a gaseous medium, then this can be used both for the direct and for the indirect cooling.
In the event that helium or gases from very-low-temperature fluids, such as liquid nitrogen, carbon tetrachloride and benzole nitride are used as first and/or second cooling fluid, a particularly good cooling effect can be achieved.